The present application is a U.S. national phase application of PCT International Application No. PCT/GB99/03930, having an international filing date of Nov. 25, 1999 and claiming priority to Great Britain Application No. 9825812.2 filed Nov. 25, 1998. The above PCT International Application was published in the English language and has International Publication No. WO 00/30734.
This invention relates to a method and apparatus for removing particulates and/or other undesirable components. In particular the invention concerns such a method and apparatus for removing particulates and/or other undesirable components from a gas stream such as an exhaust gas stream.
There are many instances when it is desirable to remove particulates from a particulate-containing gas stream. For example in many industrial processes it is desirable for gases to be cleaned of particulates before undergoing eg. chemical or physical processing.
Another instance where it is desirable to remove particulates is in the field of vacuum cleaners. Although conventional filtration techniques are capable of removing comparatively coarse dust particles from the air flowing through a vacuum cleaner, such techniques are poor at removing fine particulates, which tend to cause various ailments, including respiratory disorders, if allowed to exhaust from a vacuum cleaner into the ambient air of a room.
Another field in which particulate removal is particularly important is that of diesel engine exhausts.
Diesel engines are widely used throughout the world, particularly in heavy vehicles (trucks, buses and trains) and increasingly in automobiles. They are robust, fuel-efficient, long-lasting, and emit relatively low levels of carbon monoxides but they suffer from two major disadvantages which are causing increasing environmental concern. These are: (a) the emission of particulates, and (b) the emission of undesirable components such as oxides of nitrogen, sulphur and/or polynuclear hydrocarbons.
The particulates, which are carbonaceous in nature, are associated with undesirable hydrocarbons, of which the class known as polycyclic aromatic hydrocarbons are of particular concern. One of these compounds, 3-nitrobenzanthrone, has been reported (Suzuki et al, Environment Science and Technology, Volume 3, page 2772, 1997) as being extremely active in causing mutations in the DNA of standard strains of bacteria, as measured by the so-called Ames Test. Other compounds also present in diesel exhaust gases, such as 1,8-dinitropyrene, have also been found to be strongly mutagenic. These observations point to a strong link between diesel exhaust emissions and carcinogens in the atmosphere. It has been estimated that the tiny combustion particles, especially those with dimensions of less than 1 micrometre, are capable of carrying these chemicals into the deep recesses of human lungs. Virtually all diesel particles are in this size range (Michael P. Walsh xe2x80x9cGlobal Trends in Diesel Emission Controlxe2x80x941 1997 Updatexe2x80x9d, SAE Technical Series Paper 970179). Particulates from diesel exhaust gases may cause 10,000 deaths in Britain and 60,000 deaths in the USA each year. (xe2x80x9cDying from too much dustxe2x80x9d, New Scientist, Mar. 12, 1994, page 12). This leads to the conclusion of J. Merefield and I. Stone (New Scientist, Sep. 20, 1997, page 58) that xe2x80x9cwe could greatly improve our health and the urban air if we had better control over our vehicles"" exhaustsxe2x80x9d.
Oxides of nitrogen (and ozone) are also very undesirable atmospheric pollutants because they generate oxygen radicals, which can damage DNA and attack cell membranes. Nitrogen dioxide, NO2, emitted from diesel engines is capable of producing oxygen atoms under the influence of sunlight, i.e.:
sunlight+NO2xe2x86x92NO+O.
These oxygen atoms can then combine with oxygen in the atmosphere to form ozone, O3, i.e.:
O+O2xe2x86x92O3.
This explains why ozone pollution is especially serious during warm, sunny days. It should be noted also that ozone is harmful not only to humans in a number of ways (damage to airways linings, inflammatory reactions, and increased likelihood of asthma attacks), but also to vegetation, causing reduced yields from a range of crops including wheat, barley and peas.
Naturally, because of the worldwide concerns for these problems, there has been a great deal of effort directed towards finding a solution. The most obvious of these, designed to remove particulates, makes use of filters or traps. The main problem with these is that they tend to become blocked, which results in numerous inefficiencies in the operation of the engines to which they are attached. This technology has been studied extensively. (See, for example, Y. Teraoka et al., Catal. Today, Volume 27, page 107 (1996). It is possible to regenerate the filters by burning off the trapped soot, and this procedure is rendered more efficient if a catalyst is incorporated into the filter material (as referred to by J. P. A. Neeft et al. in Appl. Catal. B. Environmental, Volume 8, Page 57 (1996)). Naturally this constant need for removal and regeneration is a serious disadvantage. An alternative approach has been advocated by Cooper and Thoss (SAE Technical Paper 890404 (1989)). In this case a platinum-containing catalyst was mounted upstream of a particulate trap in order to oxidise nitrogen to nitrogen dioxide. The resulting NO2 is a powerful oxidising agent which is capable of removing carbon, viz.,
2NO2+Cxe2x86x922NO+CO2.
Unfortunately, as this equation shows, the reaction generates nitric oxide. Furthermore, the catalyst is sensitive to poisoning by sulphur, which is present to a greater or lesser extent in diesel fuels throughout the world.
Therefore it is clear that there is a need for a system which is capable of removing particulates efficiently (especially those smaller than 1 xcexcm) and which is unaffected by the presence of sulphur.
U.S. Pat. No. 5,453,107 and U.S. Pat. No. 3,803,813 disclose apparatus for filtering particulates from exhaust and other gases wherein the gas is first bubbled through a liquid prior to passing through a filter.
U.S. Pat. No. 5,129,926 describes an engine exhaust system comprising a water-filled scrubber tank through which the exhaust gas is released. The system further comprises a moisture trap for returning some of the condensed gas back to the inlet manifold of the engine and a filter to filter the gases once they have passed through the moisture trap.
U.S. Pat. No. 3,957,467 discloses an exhaust gas purifier and silencer in which exhaust gases are first released from a conduit into a liquid to purify the gas and thereafter returned to the same conduit and exhausted.
According to a first aspect of the invention, there is provided a method of removing particulates and/or other undesirable components from a gas stream, comprising wetting at least a portion of the gas stream in order that the particulates and/or undesirable components are entrapped and/or dissolved in the liquid thereby cleaning the gas; and further comprising cooling the gas stream and/or cleaned gas to condense desired fractions thereof.
This method is advantageously simple, and inexpensive since it does not require the expensive metals usually needed in particulate removal systems that operate by catalysts. Also, the method of the invention has the capability of removing certain oxides of nitrogen and sulphur.
Desirable the majority or substantially all the gas stream is wetted.
It is to be understood that the term wetting includes both passing said at least a portion of the gas stream through a liquid, or merely blowing the gas stream onto the liquid in order to wet it.
Preferably the liquid is or is predominantly water, and more preferably includes a detergent. In preferred embodiments the detergent constitutes 1 part in 50,000 of the liquid.
The detergent ensures that the liquid wets the fine particulates, and has been found to be particularly effective when the gas stream is diesel exhaust gas.
However, nitric oxide is only sparingly soluble in water, thus a strong oxidising agent such as ozone may be provided in order to oxidise the nitric oxide to nitrogen dioxide. The ozone may also serve to oxidise any harmful polynuclear hydrocarbons to less harmful hydrocarbons.
Additionally, or alternatively the water may contain sodium carbonate in order to convert any sulphur present in the gas stream to sodium sulphate. The liquid may include antifreeze (eg. ethylene glycol). This makes the method of the invention more suitable for use in road vehicles used in cold climates.
In one arrangement the step of wetting the gas stream occurs in a container having a splash guard for minimising fragmentation and/or loss of the liquid from the container.
This feature of the inventive method advantageously prevents the liquid from being thrown outwardly of the container under the force of the gas stream.
Conveniently the step of wetting the gas occurs in a vessel having an outlet for cleaned gas, the method including the step of cooling the cleaned gas to condense desired fractions thereof. This step ensures that any of the liquid vaporised and conveyed to the outlet with the cleaned gas is condensed and thereby available for further use in the method of the invention. This feature is particularly advantageous when the method is used to clean diesel exhaust gases, that are usually at a high enough temperature when passed through the liquid to vaporise the latter. The condensing step avoids wastage of the liquid.
Typically the cooling takes place in the cleaned gas outlet. If the cleaned gas outlet is appropriately located, the condensed liquid may flow under gravity back to the main body of liquid in the container.
The splash guard (when present) also optionally cools the cleaned gas. This may be achieved eg. by manufacturing the splash guard from a material having a comparatively high thermal conductivity. Many metals are suitable.
Preferably the gas stream flows into the liquid via a submerged pipe having a plurality of apertures defining an aggregate area at least equal to the diameter of the pipe This feature ensures that the method does not cause serious back pressure.
Alternatively the gas stream may be blown onto the surface of the liquid via a pipe. The pipe may be positioned in the container so as to induce mixing or swirling of the liquid on blowing of the gas stream onto the surface of the liquid. This can serve to ensure adequate wetting of the gas stream.
In turn this means that the method is useable to clean the exhausts of internal combustion engines, since the method can be practiced without significantly affecting the engine exhaust back pressure.
Preferably the gas stream is or includes exhaust gas from an internal combustion engine, particularly a compression ignition engine. The method of the invention may also be practiced on other gas streams, including but not limited to those mentioned herein.
The method may optionally include filtering of the liquid. This may allow a quantity of the liquid to be used several times. The invention may include the step of further passing a gas stream through the filtered liquid.
According to a second aspect of the invention, there is provided an apparatus for removing particulates and/or other undesirable components from a gas stream, comprising: a container containing a liquid; an inlet for the gas stream permitting wetting of at least a portion of the gas stream; and an outlet from the container for cleaned gas wherein the inlet and/or outlet includes condensing means for cooling and condensing desired fractions of the gas stream and/or cleaned gas. This apparatus advantageously permits practising of the method of the invention.
Conveniently the inlet for the gas stream includes a pipe, connected to a source of the gas stream, at least partially submerged in the liquid and including one or more apertures or perforations permitting passage of the gas stream through the liquid. Preferably the aggregate surface area defined by the apertures in the pipe generally equals the transverse cross-sectional area of the pipe. These features ensure that the apparatus of the invention does not adversely influence the pressure of the gas stream being supplied to it.
Alternatively the inlet for the gas stream includes a pipe, connected to a source of the gas stream, arranged so as to enable the gas stream to be blown onto the surface of the liquid.
Typically the liquid is or is predominantly water, particularly water and a detergent approximately in the ratio 1 part detergent to 50,000 parts water. The liquid may also include an antifreeze. The key features of the liquid are that it adequately wets the particulates; and that it does not react undesirably with the gas. Thus any of a range of liquids may be suitable. For example the liquid may include an oxidising agent and/or a carbonate, such as sodium carbonate in order to assist with the removal of undesirable components such as nitric oxide and/or sulphur from the gas stream. A suitable oxidising agent is ozone. Thus, in a preferred embodiment the apparatus of the invention further comprises an ozone generator for providing ozone to the container. Preferably at least a portion of the ozone is passed into the liquid.
In some instances it may not be desirable to include antifreeze in the liquid, for example to minimise cost. In such circumstances it is desirable for the pipe to comprise further perforations which extend above the surface of the liquid. Thus, should the liquid freeze, the gas can still escape from the pipe by way of the perforations above the frozen liquid surface. Once the liquid defrosts, the majority of the gas stream will pass through the liquid.
These features assist in practising of the method of the invention.
Conveniently the apparatus includes a splash guard for minimising fragmentation and/or loss of the liquid from the container. The function of this is described above.
In preferred embodiments the splash guard includes a perforated plate, especially one having plural perforations, covering or substantially covering the surface of the liquid. Conveniently the splash guard includes a wire mesh overlying the surface of the liquid. In practical embodiments the wire mesh overlies, and covers, the perforated plate.
This design of splash guard has been found to be particularly effective in limiting fragmentation (splashing) of a foaming liquid such as a water and detergent mix. If the splash guard (or part thereof) is manufactured from a material, such as a metal, having good thermal conductivity, the splash guard advantageously serves to cool any liquid splashing onto it and any gas passing through it. This tends to condense any of the liquid vaporised by heat in the gas stream. The condensed liquid falls into the main body of liquid via the perforations, and is thus made available for re-use.
Conveniently the outlet for cleaned gas includes a pipe containing a wire mesh. The wire mesh in the pipe also serves to cool and condense vaporised liquid. If the location of the pipe is correctly chosen the thus condensed liquid flows back to the main body thereof and is available for re-use.
Conveniently the apparatus includes a cooler for the outlet for cleaned gas. Preferably the cooler is or includes one or more cooling pipes surrounding or within the outlet and having flowing therein a cold fluid. The cooler assists in the condensation of the cleaned gas which may comprise vaporised liquid and thus helps to minimise evaporation of the liquid from the container.
The apparatus optionally includes for filtering of particulates from the liquid. Conveniently the container includes one or more apertures for filling it with and emptying it of the liquid, thereby permitting use of the filter remotely of the container and return of the filtered liquid to the container. These features allow the liquid to be re-used several times.
In a preferred embodiment the apparatus includes a particulate detecting device, operatively connected to monitoring apparatus, in the outlet for cleaned gas. This feature permits monitoring of the cleaned gas output, and if necessary can be used to indicate when filtering of the liquid is needed.
The dependent claims hereof set out further, optional features of the invention.